The invention relates to a focus detecting device which automatically detects the focus of a photographing lens operating on the principle that when the photographing lens is focussed on the object the contrast of the image of the object becomes a maximum.
The optical image of an object formed by a photographing lens exhibits a maximum in contrast when the image is precisely focussed. This phenomenon can be explained by the fact that the light intensity (power spectrum) of the image with respect to each spatial frequency takes the maximum value when the image is precisely focussed. Thus, if the point of maximum contrast can be detected, on the basis of the phenomenon then the focus can be detected. Accordingly, a variety of automatic focus detecting devices which operate on this principle have been known in the art.
Methods for detecting contrast data can be generally classified into two systems. In the first system, a number of microphotoelectric elements are arranged on the image forming plane of the photographing lens. Among these microphotoelectric elements, a pair of adjacent elements are selected and the difference between the photoelectric outputs is statically detected to provide a contrast signal. In the second system, the optical image is scanned mechanically or electrically and a time series signal representative of the brightness distribution of the obtained image is subjected to differentiation or other such signal processing to dynamically produce a contrast signal.
Recently, an automatic focus detecting device according to the latter system has been proposed in which a self-scanning type photoelectric element unit is employed as an optical image scanner. The self-scanning type photoelectric element unit is made up of plural microphotoelectric elements and a scanning circuit. In the automatic focus detecting device, the self-scanning type photoelectric element unit is positioned in a plane in which the optical image is formed and the photoelectric outputs of the elements are successively and electrically extracted to provide a discrete time series signal. This signal is converted into an analog waveform using a sample-and-hold circuit and a smoothing circuit after which the analog waveform is differentiated in a differentiation circuit to provide an output corresponding to the gradient of the brightness distribution of the optical image, the output being employed as the contrast signal.
The self-scanning type photoelectric element unit is advantageous in that, as it can accumulate light over an entire scanning period, a sufficiently high photo-electric output can be obtained even when the brightness is low. Also, in contrast to previous systems in which it was necessary to provide as many photocurrent-voltage conversion circuits or amplifier circuits as there are pairs of photoelectric elements, only a single photocurrent-voltage conversion circuit or amplifier can process the output signals of the photoelectric elements because a time series signal is utilized. Furthermore, the system in which the brightness distribution of an image is subjected to differentiation using a self-scanning type photoelectric element unit is advantageous in that light low in intensity can be detected owing to the charge storage effect of the elements. However, it is still disadvantageous in that the differentiation waveform is so steep that it is difficult to sample and hold the differentiation output thereof so that it is necessary to provide an intricate circuit in order to hold the differentiation output. In addition, in this device a discrete photoelectric conversion waveform is formed at each microphotoelectric element. In order to convert this waveform into an accurate analog waveform it is required to remove an unwanted high frequency component. However, in this case, a necessary high spatial frequency component of the optical image is also removed as a result of which the focus detection accuracy is decreased.
In the prior system in which the difference between the photoelectric outputs of adjacent photo-electric elements is statically extracted, unlike the latter system, it is unnecessary to provide such an intricate circuit because no differentiation waveform is employed. However, the first system suffers from a problem that in the case where it is used with a moving object or the camera is shaken, the output difference changes depending on whether the edge of the brightness distribution of the object's image is applied to a photoelectric element or not making it impossible to obtain contrast data of high accuracy.